Synchronizing mechanisms of this type, which are based on the Borg-Warner synchromesh principle, have been previously known in many different designs. A particular design of such a synchronizing mechanism has been described for instance in German Patent Specification No. 26 59 448. During shifting the gear shift sleeve is axially displaced from a neutral position entraining on this way the synchronizing ring via the spring detent means to urge it against the countercone of the clutch body. This brings the synchronizing ring and the clutch body into frictional engagement whereby the speeds of the shaft, the gear shift sleeve and the synchronizing ring on the one hand and the clutch body and that gear of the pair of gears which is to be cut in on the other hand are adapted to each other. As long as synchronism is speed in not reached, the synchronizing ring is rotated by the clutch body in the circumferential direction relative to the gear shift sleeve to the extent permitted by the form-locking engagement between the synchronizing ring and the gear shift sleeve. The synchronizing ring is provided with check faces which are thereby moved into a position in which they get into engagement with corresponding check faces provided on the gear shift sleeve to prevent any axial displacement of the gear shift sleeve towards the clutch body. In the known synchronizing mechanisms, these check faces are usually provided on an external toothing of the synchronizing ring which is identical to the external toothing of a clutch body, while the counterfaces are provided at the ends of the teeth forming the internal toothing of the gear shift sleeve.
When synchronism in speed has been reached, the gear shift sleeve can move past the check faces of the synchronizing ring to engage the external toothing of the clutch body. The contact established between the gear shift sleeve and the synchronizing ring by means of the spring detent means is simultaneously released because the synchronizing ring is permitted to follow the axial movement of the gear shift sleeve only until it comes into contact with the countercone of the clutch body. If the gear shift sleeve is to move further, as necessary when the clutch is to be engaged, the axial connection between the gear shift sleeve and the synchronizing ring must necessarily be released.
It is a drawback of these known synchronizing mechanisms that the synchronizing ring is no longer subjected to any axial force after the action of the spring detent means has been released between the gear shift sleeve and the synchronizing ring and the gear shift sleeve has in addition passed the check faces of the synchronizing ring. As a result thereof, the synchronizing ring may come free of the countercone of the clutch body when shifting, during the time interval between the release of the axial force acting upon the synchronizing ring and the establishment of the connection between the gear shift sleeve and the coupling body, and the synchronization may consequently be lost again. This may lead to shifting noise due to the fact that the teeth of the gear shift sleeve slide past the teeth of the clutch body before the gear shift sleeve and the gear can finally be brought into engagement, maybe only with some effort. The risk of such trouble being encountered is particularly great with cold transmissions where very viscous transmission oil has a high breaking effect on the gears so that the forces counteracting the synchronization are too high to be transmitted by an unloaded synchronizing ring.